Smart and VSAT for Schools

bminish

It seems that Smart have partnered with Eutelsat to deliver 2 way satellite 'broadband' to some 527 Schools that it can't reach by either it's own DSL or with Bitstream DSL

See http://www.satcoms.org.uk/satcoms_articles/articles/forum_posts.asp?TID=35

Interestingly I see that they are doing PPP over the VSAT connection. It seems likely that this would defeat the protocol optimisations and caching features that are deployed on 2 way sat systems to reduce some of the worst effects of 2 way sat latency.

I wonder if the PPP bit is a HEANET requirement or if it's Smart's own 'bright' Idea?

Glad to see that Rural schools are only getting the very best that broadband can offer.

.Brendan

vinnyfitz

Hi Brendan

Can you explain the PPP element a little more.

What is it? Why would they be including it? What are supposed to be the advantages? What do you envisage will be the disadvantages?

Thanks.

Moriarty

Point-to-Point Protocol. It's used by basically every ISP as a transport/authentication device. I don't know the particulars of why it would be bad over 2 way sat.

Urban Weigl

I am relatively certain that HEANET require the use of PPPoE.

Edit: Moriarty, 2-way satellite uses special propriatory extensions to the TCP/IP protocol, as well as caching and pre-fetching. For example, if you go to a web site, it will send just the URL to a special server on the other end, which will than fetch the entire page including all images, and send it over the satellite link in one spurt. If PPPoE is used, it may not be possible to use these extensions.

If that is the case, web pages will not load any faster than a good dialup connection. I.e. extremely slowly.

bminish

Moriarty wrote:

Point-to-Point Protocol. It's used by basically every ISP as a transport/authentication device. I don't know the particulars of why it would be bad over 2 way sat.

PPP is an end to end transport protocol, End to end protocols will not perform well over 2 way sat systems since the latency makes doing ACKs (acknowledgements that packet(s) arrived ok and the we are ready for the next packet(s) ) take a very long time

With 2 Way Sat the Sat part of the route will have about 650 mSec Round trip time (RTT) add to this any additional delays due to satellite contention and the transit time taken from Italy to HEANET and I suspect you are looking at best case RTT of 800 mSec between School X and HEANET

Traditional 2 Way VSAT systems counter some of the latency problems in a number of ways, none of which make all the issues disappear

1/ HTTP proxying. The systems I am familiar with here look at the HTTP request and pre-fetch the entire page which is then streamed from the NOC down to the End user, this makes browsing bearable on 2 Way VSAT for non secure pages. Secure pages will still be dog slow since they cannot be proxied.

2/ local caching of DNS and small files. this is usually done in the sat modem. Doing small traffic over VSAT such as DNS or sub 10k files takes a lot longer than it should because of RTT. Caching this locally speeds things up dramatically. When I was on VSAT I ran a local squid cache which was set to cache all files, this helped a lot (even though the Sat modem had a small built in cache) especially when multiple users were sharing the connection.

3/ Sat systems often mess around behind the scenes with the MTU size and stuff like ICMP packets can get lost. Upstream MTU is not always equal to downstream MTU. There are significant gains to be had by tweaking for this since most operating systems assume that the connection is relatively low latency. Another very useful Sat Tweak is being able to increase the RWIN (receive window) this is the amount of data that can be outstanding before an ACK must be sent.
http://cable-dsl.home.att.net/rwinanim.htm
Other sat tweaks and FAQs
http://www.broadbandreports.com/faq/satellite


Basically end to end PPP rules out 1,2 and quite possibly causes issues with optimising 3


.brendan

vinnyfitz

OK thanks.

I'm picking up on the reasons why you might not use PPP.
What I don't get, yet is why Smart think it has sufficient positives to make them announce it as good news in a press release?

Sponge Bob

Heanet require it.

Inside the VSAT gear each school will have its very own Cisco 17nn series router between the VSAT and the School LAN which will use PPP to launch a VPN thru the sat gear to Heanet. These routers have been configured by none other than Eircom for the schools , I am absolutely sure they will all work no problems

It was a Heanet idea to insist on VPN from the router, not an NCTE idea . VSAT gear can do its own VPN so it is not needed. Then again each VSAT operator has their own specialised VPN software which may or may not interoperate with the cisco gear.

Shucks

jwt

Sponge Bob wrote:

Inside the VSAT gear each school will have its very own Cisco 17nn series router between the VSAT and the School LAN which will use PPP to launch a VPN thru the sat gear to Heanet. These routers have been configured by none other than Eircom for the schools , I am absolutely sure they will all work no problems

If the techies can't configure a VPN connection over cisco routers they may as well give it up completely. Cisco routers using VPN in a [Cisco VPN] <-> [Cisco VPN] configuration is very likely the easiest config you'll ever have to do.

Mind you wait for some clown to use the Cisco Config GUI* to do it not realising it optimises for a low latency network.


John

*If your using that you certainly ain't configuring your router efficiently, and I defy anyone to tease out the config it generates, a bit like MS Frontpage mangles HTML.